Tool Overhangs Research Articles

Methodology of using PAAW and the Underlying Support Network of an L-PBF Part to Facilitate Machining

Abstract The Laser- Powder Bed Fusion (L-PBF) process is currently used to print geometrically complex, thin walled, metallic parts. Critical features on these parts must be machined. Unfortunately, the parts are difficult to fixture without clamping distortion, and are susceptible to machining chatter. This paper describes a methodology in which the underlying support network that is used to facilitate printing is used to mitigate these machining problems. For the first machining operation, the support network is adhered to a Photo-Activated Adhesive Workholding (PAAW) fixture to achieve five-sided cutting tool access and minimum cutting tool overhangs. Subsequently all of the critical geometry above the network is machined. In the second operation, the support network is machined away to create new geometry at the former support-solid metal interfaces. Tool paths are chosen to take advantage of the stiffness imparted by the network. The potential advantages of this approach are that it simplifies the machining sequence and reduces the total number of required operations and fixtures to two. The holding complexity of the first operation is simplified to the extent that it may be done using a general purpose, modular PAAW fixture. It does not require the printing of extra metal. In addition, it takes advantage of the inherent stiffness that the support network imparts to the main geometry of the part, thus making it easier to machine critical geometry without inducing chatter or excessive distortion. The results of a feasibility study indicate that this methodology is effective at mitigating chatter and creating geometrically accurate surfaces with a fine texture. Unfortunately, this study also reveals that stress relaxation distortion degrades the accuracy of these surfaces once the part is released from the fixture after the 2 nd operation. In addition, the machining of the network deposits a small quantity of difficult-to-clean powder sludge in the machining center.

Tool vibration in internal turning of hardened steel using cBN tool

The machining of hardened materials with hardness over 45 HRC has been an alternative to grinding since the 1970s, with the commercial availability of cubic boron nitride (cBN) and ceramic tools. However, the low toughness of these types of tool materials makes them very sensitive to damages caused by vibrations, which are critical for operations like internal turning, where the tool resembles a cantilever beam and therefore is susceptible to large deflections. This work aims to contribute to the study of tool performance in internal turning of long holes in hardened AISI 4340 steel in finishing conditions. Different machining conditions, two different tool holders (steel and carbide), and several tool overhangs were tested. The surface finish, acceleration (vibration) signals, and tool wear of cBN inserts were evaluated. The results show that vibration and the material of the tool holder may play a secondary role in the surface finish for stable turning, but the use of carbide tool holders makes the process stable for longer tool overhangs. Moreover, when the cutting becomes unstable, surface roughness is increased severely.

Influence of work material microstructure on vibrations when machining cast Ti-6Al-4V

Titanium alloys are known to produce shear-localized chips during machining, resulting in cyclic variations in cutting forces which in turn could cause severe problems with vibrations. However, at low cutting speeds and feed rates, continuous chips are formed, with an increase in both parameters favoring the transition to shear-localized chips. This transition is affected by work material microstructure, where a coarse microstructure gives anisotropic effects, e.g., when the size of alpha colonies is on the same order of magnitude as the primary cutting zone. The change in chip morphology with an increase in cutting parameters will then be dependent on the orientation of alpha colonies within the cutting zone. The microstructure of work material can show large variations depending on product form, e.g., cast, wrought, or sheet material, thus affecting whether the chip formation is isotropic or anisotropic. Other sources of variations also exist that can be found within the same component, such as segregation of alloying elements and differences in thermo-mechanical history during processing due to geometry. In this study, the interaction between work material microstructure, process parameters, and the machining system’s structural characteristics is studied. The aim is to further increase the knowledge about vibrations during machining of titanium and the role of microstructure and machining system properties. Different microstructures were produced by adding boron to cast Ti-6Al-4V material, where the resulting colony sizes gave both isotropic and anisotropic chip formation within the chosen cutting data range. The machining systems dynamic properties were varied by using different tool overhangs, thereby simulating different configurations of natural frequencies and stiffness. The results show the influence of both microstructure and machining system’s structural characteristics on the dynamic response of the system for different process parameters. This information can be used to increase robustness of machining operations taking into consideration this three-way relationship.

Influence of Cutting Tool Overhangs at Machining of Hardened Steels

Influence of Cutting Tool Overhangs at Machining of Hardened Steels

Effects of tool overhang on selection of machining parameters and surface finish during diamond turning

In precision machining leading to nano-metric surface finish, selection of the suitable machining parameters is a critical task. To ensure the desired surface quality, one needs to optimally select the machining parametric matrix. Towards this effort, this paper adds another critical parameter in terms of tool overhang. A well-defined set of machining exercises is carried out with different tool overhangs and machining parameters. In this investigation, an attempt has been made to locate the optimum range of tool overhang with minimum tool vibrations. The interaction between tool overhang with other parameters is also thoroughly investigated. Another important focus of this study is to find out the optimum machining parameters for the situations where it is not possible to select an optimum tool overhang. One such situation occurs when a steep concave parabolic surface needs to be fabricated. In this case a large tool overhang has to be selected. Power spectral density distribution analysis of surface roughness for different tool overhangs is performed to find out significant parameters and their degree of contribution to surface roughness. Analysis of variance is also applied to ascertain statistically significant factors contributing to surface roughness. To model the surface roughness, response surface methodology is being used. The model has been verified by conducting a series of experiments and a steep concave parabolic surface is developed by following the predictions of the developed model.

Regenerative vibration avoidance due to tool tangential dynamics in interrupted turning operations

Abstract Linear stability models are often used to predict regenerative vibrations in turning representing continuous operations, simple cutting geometries with constant coefficients and/or dominant modes acting in the feed direction. However, turning of components with interrupted features, such as turbine cases, may lead to large tool overhangs with vibration motions in the cutting speed direction and tool cut-off periods that result in the latter approaches being insufficient. This paper proposes a stability model for chatter in interrupted turning when the dominant vibration is orthogonal to the chip section plane. The method requires the calculation of a dynamic displacement factor that depends on the tool vibration frequency. The simulations of the model are supported by experimental tests for different contact fractions.

Coupled interaction of dynamic responses of tool and workpiece in thin wall milling

Chatter free thin wall machining requires knowledge of the dynamics of a machine-tool system and workpiece either for designing damping solutions or for modelling impact dynamics. Previous studies on thin wall milling mostly focussed on stability studies. However studies on the interaction between the tool and workpiece responses in thin wall machining are scarce in the literature. In this work, the coupled dynamic response of tool and workpiece is presented both for an open (thin wall straight cantilever) and for closed (thin wall ring type casing) geometry structures. Experiments were carried out for different tool overhangs and depths of cut and the machining vibration signal was analysed in time–frequency domain to study the interaction, i.e. coupling, of tool–workpiece dynamic response at various cutting tooth engagement/idle times. The findings from this study highlight the importance of tool's frequency, particularly torsional and first bending modes, in impact dynamics of thin wall milling. Moreover, the differences in dynamic response interaction between a cutting tool and thin wall plate and a cylinder are identified. While the analysis of the open geometry structure showed the presence of tool and workpiece responses for any depth of cut, results on closed geometry structure exhibited a complete dominance of tool mode at higher depths of cut. These findings are of critical importance in understanding the impact dynamics in thin wall milling and also of effectiveness of passive damping solutions.

Correlating surface roughness, tool wear and tool vibration in the milling process of hardened steel using long slender tools

High speed milling is an operation frequently used in finishing and semi-finishing of dies and molds. However, when it is necessary to produce molds with deep cavities and/or with small corner radius, long tools with small diameters are required. This represents a challenge for manufacturing professionals: how to minimize tool vibration using a tool with such low rigidity and obtain good workpiece surface quality and long tool lives. This paper attempts to answer this question. Milling experiments on hardened AISI H13 steel were carried out using integral and indexable insert tools with different tool overhangs and different diameters. Tool wear, workpiece surface roughness and cutting forces were measured and these parameters were correlated with the frequency response function (FRF) obtained with the tools fixed in the machine tool. The main conclusion of this study is that good workpiece surface roughness allied to long tool lives for long tools with small diameters can be achieved, provided the tooth passing frequency used in the milling process (and its harmonics) does not produce high FRF values.

Experimental Investigation of Effect of Tool Length on Surface Roughness during Turning Operation and its Optimization

In the turning operation, vibration is a frequent problem, which affects the result of the machining and in particular the surface finish. Tool life is also influenced by vibrations. Severe acoustic noise in the working environment frequently results as a dynamic motion between the cutting tool and the work piece. In all cutting operations like turning, boring and milling vibrations are induced due to deformation of the work piece. In the turning process, the importance of machining parameter choice is increased, as it controls the surface quality required. Tool overhang is a cutting tool parameter that has not been investigated in as much detail as some of the better known ones. It is appropriate to keep the tool overhang as short as possible; however, a longer tool overhang may be required depending on the geometry of the work piece and when using the holeturning process in particular. In this study, we investigate the effects of changes in the tool overhang in the external turning process on both the surface quality of the work piece and tool wear. For this purpose, we used work pieces of AISI 1050 material with diameters of 20, 30, and 40 mm; and the surface roughness of the work piece were determined through experiments using constant cutting speed and feed rates with different depth of cuts (DOCs) and tool overhangs. We observed that the effect of the DOC on the surface roughness is negligible, but tool overhang is more important. The deflection of the cutting tool increases with tool overhang. Two different analytical methods were compared to determine the dependence of tool deflection on the tool overhang. Also, the real tool deflection values were determined using a comparator. We observed that the tool deflection values were quite compatible with the tool deflection results obtained using the second analytical method.

Modeling and Optimization of Vibration Amplitude in Turning of Stainless Steel AISI 304 at Various Tool Overhangs

Chatter is detrimental to all machining operations. In metal turning operations it leads to inferior surface topography, reduced productivity, and shortened tool life. Various researchers have thus tried to develop mathematical models and theories for chatter formation in order to find optimum ways of chatter suppression. This research investigated the chatter phenomenon in turning of stainless steel AISI 403 in the light of vibration amplitude. Central composite rotatable design of experiments under Response Surface Methodology (RSM) was employed to create a second order mathematical model, and the adequacy of the model was verified using analysis of variance (ANOVA). Machining operations were carried out on a conventional engine lathe (Harrison M390, England) with rated power of 5.5 kW and maximum spindle speed of 2000 rpm. Cemented Tungsten Carbide (WC-Co) inserts were used to machine a stainless steel cylinder. The data acquisition system consisted of a vibration sensor (KISTLER accelerometer Type 8774A50) and a signal conditioning unit, which was connected to the computer via a data acquisition (DAQ) card. Analog input signals were fed into the DAQ card and evaluated using the DASYLab 5.6 software. The vibration results were analyzed in the frequency domain (FFT) plots. It was observed that the uncontrolled vibration of the tool holder was the largest contributor to chatter formation.

The dependence of tool overhang on surface quality and tool wear in the turning process

Intheturningprocess,theimportanceofmachining parameter choice is increased, as it controls the surface quality required.Tooloverhangisacuttingtoolparameter thathasnot been investigated in as much detail as some of the better known ones. It is appropriate to keep the tool overhang as short as possible; however, a longer tool overhang may be required depending on the geometry of the workpiece and when using the hole-turning process in particular. In this study, we investigate the effects of changes in the tool overhang in the external turning process on both the surface quality of the workpiece and tool wear. For this purpose, we used workpieces of AISI 1050 material with diameters of 20, 30, and 40 mm; and the surface roughness of the workpiece and tool wear were determined through experiments using constant cutting speed and feed rates with different depth of cuts (DOCs) and tool overhangs. We observed that the effect of the DOC on the surface roughness is negligible, but tool overhang is more important. The deflection of the cutting tool increases with tool overhang. Two different analytical methods were compared to determine the dependence of tool deflection on the tool overhang. Also, the real tool deflection values were determined using a comparator. We observed that the tool deflection values were quite compatible with the tool deflection results obtained using the second analytical method.

Lapping Simulation Based on the Gap Theory (1st Report)

The simulation of lapping to correct the surface profile of a workpiece has been made very little study. In contrast, the simulation of polishing has been developed using the gap theory and it has been demonstrated that there is good agreement between simulated and experimental results. Our study investigates the application of polishing simulation to lapping. We have reported that a lapped profile can be calculated if a suitable value of relative elastic coefficient is used. This paper describes workpiece and tool profiles lapped with a small tool, and corresponding simulation results calculated using the gap theory, for oscillation lapping to planarize a quartz glass wafer. Under non-oscillation conditions, the experimental results for workpiece and tool profiles correspond roughly with simulated results. When the tool overhangs the workpiece, to a significant extent, the experimental results do not correspond with the simulated ones. Therefore, the slope of the pressure distribution for the simulation was adjusted to increase rapidly when the amount of overhang is more than 30 % of tool diameter. As a result, the experimental and simulation results corresponded very closely. In uniform-oscillation-speed lapping, it was demonstrated that the achieved workpiece and tool profiles coincide approximately with the results of the simulation.

Oscillation-Speed-Control-Type Polishing with a Small Tool (3rd Report)

As ULSI feature dimensions continue to shrink, wafer flatness of less than 0.1μm has become essential, and any flaws in surface topography cause serious problems in device patterning. CMP has become a standard technology to planarize surface topography. We have developed an oscillation-speed-control-type sequential grinding and polishing machine and have simulated the polishing process. A previous paper showed that experimental profiles polished with normal and reverse rotations agreed well with the simulated result calculated with a modified equation of relative velocity. However, the simulation broke down when the tool overhung the wafer by a significant amount. This paper presents measured pressure distributions for various tool overhangs, and a method to compensate for the difference between theoretical and experimental distributions, and polishing amounts produced using the compensation. The pressure distribution from air pressure in an air-bag is significantly different than that from a weight, and its characteristics depend on the machine. The profile of the pressure distribution for large overhangs becomes concave rather than linear. The simulated results with concave pressure distributions agreed well with experimental results.

小形工具による揺動速度制御研磨加工 第２報 高速・はみ出し加工時における形状

As ULSI feature dimensions continue to shrink, and the number of interconnect layers increases from 4 to 8, the resulting surface topography causes serious problems in device patterning. CMP has become a standard technology used in semiconductor manufacturing to planarize the surface topography. We have developed an oscillation-speed-control-type sequential grinding and polishing machine and have proceeded to simulate the polishing process. A previous paper showed theoretically that it is possible to polish within±1% uniformity with the optimum oscillation speed control. This paper presents removal rates measured under high relative velocity and high pressure, and a comparison between experiment and simulation for polishing profiles. It is shown that the removal rate begins to saturate gradually from low relative velocity and that Preston's equation can be used under high pressure caused when the tool overhangs from the wafer. In addition, the experimental result of profiles polished at normal and reverse rotations with disc and ring tools agreed well with the simulation result calculated with a modified equation of relative velocity under high velocity. However, it did not agree when the tool overhung the wafer by a significant amount.

揺動速度制御型連続研削・研磨装置の開発 第１報 装置構造と基本研磨特性

ULSI devices, which consist of fine patterns of less than 0.1 μm, require a large silicon wafer having a site flatness better than 0.1 μm/30mm. We have developed an oscillation-speed-control-type sequential grinding and polishing machine, which can accurately polish a 12-inch wafer to less than 0.3 μm in flatness by measuring wafer profile on machine after highly accurate grinding. This paper describes the structure and motion accuracies of the machine, the basic polishing characteristics, and the polishing profiles at uniform oscillation speed. Pressure and stock removal increase in the periphery of a wafer when the tool overhangs from the wafer. They agreed with the theoretical results. Super-high flatness of less than 0.2 μm over the entire surface was achieved by adjusting the oscillation range at a uniform oscillation speed. This result is consistent with a simulation corrected on the basis of an experimental result that shows that stock removal saturates with the increase of relative velocity.